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Journal ArticleDOI

Particle breakage during cyclic triaxial loading of a carbonate sand

06 Jan 2009-Geotechnique (Thomas Telford Ltd)-Vol. 59, Iss: 5, pp 477-482
TL;DR: In this paper, the results of a number of drained cyclic triaxial tests on loose, uniformly graded samples of Dogs bay carbonate sand were presented and it was observed that particle breakage is dependent on stress level, cyclic stress ratio, and creep and is directly related to volumetric strain.
Abstract: Dynamic loading of embankment, foundation and pavement structures results in particle breakage of the constituent granular materials, when the stresses imposed on their particles exceed their strength. This paper presents the results of a number of drained cyclic triaxial tests on loose, uniformly graded samples of Dogs bay carbonate sand. It is observed that particle breakage is dependent on stress level, cyclic stress ratio, and creep and is directly related to volumetric strain. Drained cycling increases volumetric strain and therefore more breakage occurs when larger numbers of cycles are imposed. The increase in particle breakage from one cycle to the next indicates that while particles may not be loaded to their full capacity in a given cycle, they can be crushed in subsequent cycles without any variation in the amplitude of cyclic loading.

Summary (1 min read)

MATERIALS AND METHODS

  • Loose carbonate sands of biogenic origin tend to crush more easily than silica based sands, as their fragile shell particles fracture (Coop, 1990).
  • In the current study the sand was sampled from the intertidal zone.
  • Each of the cyclic tests were drained, and all of the tests were designed to have cycles of loading symmetrical about the isotropic axis with a cyclic stress ratio β * (= Δqmax/p'i), usually of 20%.
  • For this study material retained in the British Standard 0.063mm sieve was taken as the minimum particle size below which breakage was deemed insignificant.

RESULTS

  • The volumetric strain behaviour of the tests undergoing cyclic loading are shown in Figure 3(a, b and c) for tests undergoing 150, 1000 and 5000 cycles respectively.
  • Pa experienced more than twice the volumetric strain of those tests at 500kPa.
  • Where 1000 cycles of loading were applied ), it is apparent that a significant portion (approx. 60 - 70%) of the cyclic-induced volumetric strain develops over the initial 100-150 cycles.
  • A single test (Dogs 8) was carried out to examine the effect of creep on the observed response.

Quantification of Particle Breakage

  • The particle size distribution tests at each stress level are presented in Figure 4.
  • The relationship is similar to that shown in Figure 3 for volumetric strain, with significantly more breakage occurring in the first 150 cycles (approx. 55 - 70%), in comparison with the subsequent 850 cycles.
  • As shown a clear relationship exists between these two parameters.
  • As illustrated in this figure, the measurements of particle breakage from the current study compare well with their isotropically compressed tests, despite the uniform grading used here.
  • As discussed above, increasing the number of cycles results in greater particle breakage, and therefore at a given p' the Br values observed plot further from the NCL, as estimated by Coop & Lee (1993).

CONCLUSIONS

  • This paper has presented the results of a number of drained cyclic triaxial tests on loose, uniformly graded samples of carbonate sand, which were firstly isotropically compressed to a specified stress level.
  • It was observed that particle breakage is dependent on stress level, cyclic stress ratio, and creep and is directly related to volumetric strain.
  • Drained cycling increases volumetric strain and therefore more breakage occurs when larger numbers of cycles are imposed.
  • The findings of this research can be qualitatively related to the earlier DEM simulations of O’Neill (2005).
  • The micromechanical explanation (informed by O’Neill’s work) is that the contact force network evolves from one cycle to the next, with different particles participating in the most highly loaded branches of the contact force network from cycle to cycle.

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Title Particle breakage during cyclic triaxial loading of a carbonate sand
Authors(s) Donohue, Shane; O'Sullivan, Catherine; Long, Michael (Michael M.)
Publication date 2009-01-06
Publication information Géotechnique, 59 (5): 477-482
Publisher Thomas Telford Ltd.
Item record/more information http://hdl.handle.net/10197/4892
Publisher's version (DOI) 10.1680/geot.2008.T.003
Downloaded 2022-08-09T15:43:30Z
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1
Title of paper: Particle breakage during cyclic triaxial loading of a
carbonate sand
Names of authors: Dr. Shane Donohue
1
, Dr. Catherine O’Sullivan
2
, Dr.
Michael Long
1
Affiliation of authors: 1 School of Architecture, Landscape and Civil Engineering,
University College Dublin (UCD)
2 Dept. of Civil and Environmental Engineering, Imperial
College London
Contact address: Shane Donohue, School of Architecture, Landscape and
Civil Engineering, University College Dublin (UCD),
Newstead building, Belfield, Dublin 4, Ireland.
Phone: +353-87-9711917
Fax: +353-1-7163297
e-mail: shane.donohue@ucd.ie
Keywords: calcareous soils, laboratory tests, sands
INTRODUCTION

2
Dynamic loading of embankment, foundation and pavement structures results in particle
breakage of the constituent granular materials, when the stresses imposed on their particles
exceed their strength. Particle breakage is dependent on a number of “macro-scale”
parameters (e.g. stress level, grading, void ratio) as well as on the characteristics (e.g. size,
shape, strength, mineral composition) of the constituent particles (Hardin 1985; Coop
1990; Lade et al. 1996; McDowell & Bolton 1998; Nakata et al. 2001; Coop et al. 2004;
Leleu & Valdes 2007). A number of recent detailed studies have considered the
relationship between particle crushing and soil response. For example, Coop et al (2004)
demonstrated a link between particle breakage and volumetric compression, and showed
using a series of ring shear tests that particle crushing during shearing continues to very
large strains. Cheng et al (2003) used DEM (discrete element modelling) to demonstrate a
link between particle crushing and response along the normal compression line, while
Lackenby at al. (2007) studied the effect of confining pressure on ballast degradation
under cyclic triaxial loading. Particle breakage is generally quantified through a
comparison of pre and post crushing particle size distribution curves (Hardin 1985).
Photoelasticity studies (e.g. Drescher & De Josselin de Jong, 1972) and DEM
simulations (e.g. Cundall & Strack 1979) have both demonstrated that the distribution of
stress in a granular material is highly heterogeneous. Particles do not equally share in the
bearing of load, and those with highly loaded contacts are usually aligned in chains or
columns. O’Neill (2005) simulated a series of strain controlled cyclic biaxial tests on a 2D
granular material using PFC2D (Itasca, 2002), some of the results of which are presented
in Figure 1. The specimen configuration considered by O’Neill, included 7771 circular
(disk) shaped particles with diameters varying between 2 and 3 mm (Figure 1a). Following
isotropic compression a series of strain controlled load cycles were applied to the
specimen. In each cycle the axial compressive strain was increased to ε
a
= 1% and then

3
reduced to ε
a
= 0%. The distribution of interparticle forces though the specimen at ε
a
=
1% is shown for the first and fourth load cycles in Figures 1(b) and 1(c) respectively. To
illustrate both the magnitude and orientation of the interparticle contact forces in the DEM
simulations, a line is drawn between the centroids of contacting particles, the thickness of
which is weighted to be proportional to the magnitude of the compressive force between
these particles. This approach is often used to visualise force chains in granular materials
(e.g. Thorton and Barnes (1986)). As shown the force network is not the same in both
cases, i.e. different particles are experiencing the largest forces from cycle to cycle. We
can reasonably anticipate that degradation in the form of breakdown of asperities and
particle crushing is associated with particles participating in the strongest force chains
(McDowell et al. 1996; Cheng et al. 2001; Vallejo et al. 2006). If these force chains are
not static as shown by O’Neill (2005), then additional damage to the material should occur
in each cycle.
The objective of the current study is to consider a realistic granular material known
to exhibit crushing and explore the degradation of this material during drained cyclic
triaxial loading by quantifying particle breakage. The study considers the sensitivity of the
response both to the mean effective stress (p') value and the number of cycles of loading.
MATERIALS AND METHODS
Loose carbonate sands of biogenic origin tend to crush more easily than silica based sands,
as their fragile shell particles fracture (Coop, 1990). The crushing of these materials may
occur even at relatively low pressures (Coop & Lee, 1993), thereby facilitating research
into particle crushing. In the current study the biogenic carbonate sand considered was
obtained from Dogs Bay on the west coast of Ireland. Dogs Bay sand consists of angular
mollusc, gastropod and forminifera fragments (Golightly, 1989). As a result of the open

4
structure created by these angular particles, the sand has a high initial void ratio. The
material is poorly graded (Figure 2), with a carbonate content of 88-94% (Houlsby et al.
1988). The engineering behaviour and properties of the material have been examined in
detail by a number of researchers (e.g. Houlsby et al. 1988; Golightly 1989; Coop 1990;
Coop & Lee 1993; Yasufuku & Hyde 1995; Hyodo et al. 1998; Coop et al. 2004;
Tarantino & Hyde 2005; Qadimi & Coop 2007). There is some variability among the
particle size distributions (Figure 1) and index properties (Table 1) reported in the
literature. These variations are likely due to different sampling locations at Dog’s Bay. For
example Golightly (1989) and Coop et al. (1993) used sand from the dunes, whereas
Tarantino & Hyde (2005) used material from the intertidal zone. In the current study the
sand was sampled from the intertidal zone.
It has been shown that particle breakage is greater for uniformly graded than for
well graded sands (Nakata et al., 2001; Coop et al. 2004). Therefore in order to maximise
breakage potential tests were performed on material from the 300–425μm sieve interval.
This sieve interval was also used by Coop et al. (2004) in their ring shear tests. Due to the
highly crushable nature of the material, wet sieving was necessary to ensure that additional
damage did not occur during preparation. As shown in Table 1, e
max
is considerably higher
for this uniform grading than the natural grading. Samples of 50mm diameter were
prepared by pluviation into a water-filled membrane, held within a mould on the triaxial
pedestal. This approach was used to provide very loose samples which were both
consistent and comparable. The soil, which was first submerged under water and placed
under a vacuum, to assist saturation, was transferred to the membrane in a spoon (also
under water). The triaxial tests were conducted using a GDS computer-controlled Bishop
& Wesley system, which had a maximum cell pressure of 1700kPa.

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TL;DR: The distinct element method as mentioned in this paper is a numerical model capable of describing the mechanical behavior of assemblies of discs and spheres and is based on the use of an explicit numerical scheme in which the interaction of the particles is monitored contact by contact and the motion of the objects modelled particle by particle.
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Frequently Asked Questions (2)
Q1. What are the contributions in this paper?

In this paper, the results of a number of drained cyclic triaxial tests on loose, uniformly graded samples of carbonate sand were firstly isotropically compressed to a specified stress level. 

However, further micro-mechanics studies are needed before a definite theory can be proposed to explain the mechanics underlying the observed decrease in the rate of volumetric strain and particle crushing as cyclic loading progresses.